1. Field of the Invention
The present invention relates to image sensor modules and more particularly, to an image sensor module having precise image-projection control.
2. Description of the Related Art
An image sensor module applicable to a camera device used in a cell phone, PDA, or any of a variety of portable electronic products is generally comprised of an image sensor chip, which is made through an image-sensor technology of the semiconductor manufacturing, and an optical lens assembly. When the optical lens assembly projects an object onto the image sensor chip, the circuit operation of the image sensor chip accurately picks up the projected image and passes the image signal of the object to the aforementioned portable electronic product. The quality of the image projection of the optical lens assembly determines the quality of the image signal processed by the image sensor chip. If the optical lens assembly manufactured by a module engineering of stacking and joining lenses together cannot focus a projected image onto the image sensor chip precisely, an out-of-focus problem will occur, thereby degrading the image quality.
FIG. 1 is an exploded view of a focus adjustable type image sensor module according to the prior art. According to this design, the image sensor module 1 comprises a screw body 10, a sensor chip 11, and a lens assembly 12. The image sensor chip 11 is fixedly mounted inside the body 10, having an image pick-up element 110 on the position aligned with the direction of a main optical axis. The body 10 has a transparent substrate 101 and a threaded inside wall 102. The transparent substrate 101 and the threaded inside wall 102 define an accommodation chamber for accommodating the lens assembly 12. The lens assembly 12 has an optical lens 120, and a threaded outside wall 121 threaded into the threaded inside wall 102 of the body 10. By means of adjusting the depth of the lens assembly 12 inside the accommodation chamber with rotating along the threading path of the threaded inside wall 102 of the body 10, optical aberrations caused by the lens 120 module engineering of the formation of the lens assembly 12 are controlled, so that the lens 120 accurately focuses the image onto the image pick-up element 110. However, the friction force that is produced during rotation of the lens assembly 12 may cause fine particles to fall from the interface between the threaded inside wall 102 of the body 10 and the threaded outside wall 121 of the lens assembly 12 to the transparent plate 101. As a result, impurities may block the optical path to lower the quality of the image projection of an object. Further, the threaded inside wall 102 of the body 10 must have a certain height so that the lens assembly 12 can be adjusted vertically therein. Thus, the image sensor module 1 cannot fit the requirement for a small-sized portable electronic product.
FIG. 2 shows a stacking type of image sensor module made by semiconductor wafer manufacturing according to the prior art. According to this design, the image sensor module 2 comprises a silicon wafer 21, a first spacer plate 22, a first cover plate 23, a first lens module 24, a second spacer plate 25, a second lens module 26, a third spacer plate 27, and a second cover plate 28, which are stacked in sequence. The spacer plates 22, 25 and 27 are made of transparent glass with grinding and polishing to a predetermined thickness, and respectively provided with openings for passing image forming light rays from the lens modules 24 and 26, to the silicon wafer 21. The stacked structure of the image sensor module 2 is than cut into multiple image sensor elements 20. As shown in FIG. 3, each image sensor elements 20 comprises an image sensor chip 210, a first spacer 220, a first cover 230, a first lens 240, a second spacer 250, a second lens 260, a third spacer 270, and a second cover 280. Also a sticky resin material is provided then hard-baked between each two adjacent layers of the image sensor module 2 thus forming multiple adhesive layers 200 of the image sensor element 20. Therefore, the optical distance of the image forming light rays from the lenses 240 and 260 to the projected image on the image sensor chip 210 is controlled by the thickness of the spacers 220 and 250 and the adhesive layers 200. Practically, there is about 5% error in the grinding process of forming the spacers 220, 250 and 270, and more than 20% error in the film thickness control of the resin material for the adhesive layers 200. Under the factor of multi-layer manufacturing errors, it is difficult to accurately focus the projected image by the lenses 240 and 260 onto the image sensor chip 210, thereby degrading the image forming quality of the image sensor element 20.